File I/O (Input/Output)

File I/O (Input/Output) in C. This allows you to read from and write to files, which is essential for many applications.

File I/O in C

Opening and Closing Files

To work with files, you first need to open them using the fopen function and close them with fclose.

Syntax for fopen:

FILE *fopen(const char *filename, const char *mode);

• filename: Name of the file.

• mode: File access mode (e.g., "r" for reading, "w" for writing).

Syntax for fclose:

int fclose(FILE *stream);

• stream: Pointer to the FILE object.

Example: Opening and Closing Files

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "w");  // Open file for writing
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    // Write to the file
    fprintf(file, "Hello, World!\n");
    
    // Close the file
    fclose(file);
    
    return 0;
}

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Reading from Files

To read data from a file, use functions like fscanf, fgets, and fread.

Syntax for fscanf:

int fscanf(FILE *stream, const char *format, ...);

• stream: Pointer to the FILE object.

• format: Format string similar to printf.

Syntax for fgets:

char *fgets(char *str, int n, FILE *stream);

• str: Buffer to store the string.

• n: Maximum number of characters to read.

• stream: Pointer to the FILE object.

Syntax for fread:

size_t fread(void *ptr, size_t size, size_t count, FILE *stream);

• ptr: Pointer to the memory where data will be read.

• size: Size of each item to read.

• count: Number of items to read.

• stream: Pointer to the FILE object.

Example: Reading from a File

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");  // Open file for reading
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    char buffer[100];
    
    // Read the file line by line
    while (fgets(buffer, sizeof(buffer), file)) {
        printf("%s", buffer);
    }
    
    // Close the file
    fclose(file);
    
    return 0;
}

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Writing to Files

To write data to a file, use functions like fprintf, fputs, and fwrite.

Syntax for fprintf:

int fprintf(FILE *stream, const char *format, ...);

• stream: Pointer to the FILE object.

• format: Format string similar to printf.

Syntax for fputs:

int fputs(const char *str, FILE *stream);

• str: String to write.

• stream: Pointer to the FILE object.

Syntax for fwrite:

size_t fwrite(const void *ptr, size_t size, size_t count, FILE *stream);

• ptr: Pointer to the data to write.

• size: Size of each item to write.

• count: Number of items to write.

• stream: Pointer to the FILE object.

Example: Writing to a File

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "w");  // Open file for writing
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    // Write to the file
    fprintf(file, "Hello, File I/O!\n");
    fputs("Writing using fputs.\n", file);
    
    // Close the file
    fclose(file);
    
    return 0;
}

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Error Handling

Error handling is crucial for robust file operations. Use ferror and clearerr to check and clear errors.

Syntax for ferror:

int ferror(FILE *stream);

• stream: Pointer to the FILE object.

Syntax for clearerr:

void clearerr(FILE *stream);

• stream: Pointer to the FILE object.

Example: Error Handling

#include <stdio.h>

int main() {
    FILE *file = fopen("nonexistent.txt", "r");  // Try to open a non-existent file
    
    if (file == NULL) {
        perror("Error opening file");  // Print error message
        return 1;
    }
    
    // Perform file operations...
    
    if (ferror(file)) {
        printf("Error reading file\n");
    }
    
    clearerr(file);  // Clear error indicators
    
    fclose(file);
    
    return 0;
}

Explanation:

• perror: Prints a descriptive error message based on the current value of errno.

• ferror: Checks if an error occurred on the file stream.

• clearerr: Clears the error indicators for the file stream.

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Summary

• Opening and Closing Files: Use fopen and fclose.

• Reading Files: Use fscanf, fgets, and fread.

• Writing Files: Use fprintf, fputs, and fwrite.

• Error Handling: Use ferror, clearerr, and perror to handle and report errors.